1. Field of the Invention
The invention relates to flight management systems for an aircraft and more specifically to lateral guidance in area navigation systems.
2. Description of the Prior Art
Aircraft area navigation systems in which a navigation computer guides an aircraft along a flight plan through a series of point-to-point transitions from an inbound course or leg of a waypoint to the next leg or outbound course thereof are known in the prior art. In U.S. Pat. No. 3,994,456, issued Nov. 30, 1976 to William C.; Post and Edmond E. Olive, entitled "Steered Lateral Course Transition Control Apparatus for Aircraft Area Navigation Systems," assigned to the assignee of the present invention, an area navigation system controlled the aircraft to follow a circular transition path from a first straight line course to a second straight line course through predetermined waypoints. An alternative prior art transition control apparatus may be found in U.S. Pat. No. 4,354,240, issued Oct. 12, 1982 to Edmond E. Olive, entitled "Flight Path Transition Control Apparatus with Predictive Roll Command," also assigned to the assignee of the present invention. In this area navigation system the transition control apparatus executes an exponential transition path from a first course to a second course, thereby avoiding undesirable overshoot of the new course experienced in the aforesaid Post, et al patent.
The above described transition methods do not preclude the aircraft from violating airway boundaries and may result in excessive oscillatory motions resulting in passenger discomfort. The Federal Aviation Administration (FAA) specifies a route width of protected airspace that cannot be violated while navigating along the flight plan. Typically the specified route width is four nautical miles on each side of the leg en route, and two nautical miles on each side of the leg in terminal areas. In the controlled environment of present air traffic, most flight plans consist of several legs or segments starting at the origin of the flight and ending at the destination of the flight plan. In order for the aircraft to fly from the origin to the destination it is necessary for the aircraft to steer from the first track to the next at a predetermined distance from the way point and permit steering signals to capture the next leg by either manual pilot control through the flight director or by applying the steering signals to the automatic flight control system, as programmed by the flight management system. The current method of transitioning between legs uses curved paths which are tangent to both of the adjoining legs. This transition method is defined in the above cited U.S. Patents.
When a leg of a flight plan is short enough so that these curved path transitions overlap, there may occur a discontinuity in the desired path of the aircraft. FIG. 1 shows a typical flight plan, wherein the aircraft is transitioning between first, second, and third waypoints 110, 112 and 114. An aircraft 116 initially approaches a northerly oriented course 118 and at point 120 enters a circular flight path 122 which is tangential to the course 118. This circular flight path results in a tangential intersection with the easterly course 124 at waypoint 112. In accordance with the prior art, a further circular path 126 for transitioning between waypoints 110 and 114 is tangent to the easterly course 124 at a point 128 and to a northeasterly course 130 at point 132. It may be seen that the two flight paths 122 and 126 do not intersect and therefore the aircraft is caused to use excessive and oscillatory bank angles to maneuver through the turn at waypoint 112. This problem of the "short leg" (i.e., the distance between waypoints 110 and 112) is not recognized by the prior art. While FIG. 1 refers to a flight plan wherein the aircraft transitions through two opposing turns, FIG. 2 shows a similar problem where the flight path is in the same general direction through waypoints 134, 136, and 138. Here the aircraft 116 enters a first circular path 140 between the northerly course 118 and the easterly course 24 and is required to transition between waypoints 134 and 138 through waypoint 136. Again, leg 135 is too short to permit a smooth transition between flight paths 140 and 142 resulting in a discontinuity on the short leg.
In FIG. 3 and FIG. 4 the solution conventionally applied to the above problem by the prior art navigation systems is shown for the cases of the opposite direction and same direction course changes, respectively. In flight path 144 the short leg 124 is effectively deleted and the aircraft flies the most direct route to the leg following the short leg. This then requires open loop capture to the following leg in which control is referenced to the leg 118 preceeding the short leg 124 and then referenced to the leg 130 following the short leg but is never referenced to the short leg. Similarly, control is referenced from leg 118 in FIG. 4 to leg 137 without referencing to the short leg 135. Thus, this method imposes an extra work load on the pilot and because the aircraft is in open loop control during the transition through the short leg, the aircraft may violate airway boundaries prescribed by the FAA. In particular geometries with large course changes, deleting the short leg can result in turns in the wrong direction. Further, because there is a discontinuity in the defined flight plan, the resulting bank angles may be excessive and oscillatory.
The present invention provides an apparatus for transitioning on and off of the short leg that maintains the aircraft in protected air space, minimizes the bank angle for passenger comfort, keeps the bank angle from oscillating, and enables the pilot to maintain cognizance of the aircraft position throughout the flight plan. The invention does not delete the short leg, so that the sequencing looks normal to the pilot on his flight instruments. The invention determines which flight plan legs are short and then recalculates the transition parameters so that there are no discontinuities in the flight path. The invention uses curved path transitions which are geometrically based and guaranteed to be within the airway boundaries. The commanded turns will always be in the correct direction,